For many years labels have been bonded to containers or other objects using adhesive systems comprising aqueous solutions or suspensions of natural product binders (bonding agents) such as starches, dextrins, gums, proteins, casein and casein derivatives. These adhesive systems maintained an adequate bond if kept dry. However, labels bonded with many water-based adhesive systems could be removed by substantial humidity or immersion in water. One class of common label adhesives are the "jelly gum" adhesives. Jelly gum adhesives commonly comprise starch, urea, and a strong base such as sodium or potassium hydroxide. Optionally jelly gum adhesives can contain hydrophilic fortifying polymers in order to improve machinability, bond strength, bonding time and cost. Fortified "jelly gum" adhesive systems are taught in U.S. Pat. Ser. No. 3,857,803. These adhesives provide strong, quick adhesion between the label and the object or container, and provide adequate resistance to debonding when exposed to humidity or water, but can cause problems if the label must be quickly and easily removed from the bottle.
Labels bonded to returnable bottles using fortified jelly gum adhesives are not easily removed by water. More severed conditions such as steam or strong base (a source of aqueous OH.sup.-) are commonly used. The fortified jelly gums when exposed to caustic can be resistant to debonding since the caustic can gel the starch, reducing its water solubility. This problem can be severe in breweries where substantial quantities of bottles are returned for cleaning and refilling. Bottles having labels which are hard to remove must be often handcleaned, substantially increasing the expense of returning the refilled bottle to the marketplace.
A second class of common label adhesives comprises casein based aqueous adhesive systems. Casein and its derivatives have been used since it forms strong bonds, is humidity and water resistant, and can debond in the presence of strong base. However, in recent years casein has become more expensive.
Clearly a substantial need has arisen for a new adhesive which can be a substituent for casein and which can provide strong bonds, can be easily machined, can be resistant to humidity and water, but can quickly debond when exposed to strong base (hydroxyl, OH.sup.-).